Rotational moulding (or rotamoulding) is a process for manufacturing hollow plastics products in which a powdered thermoplastic material, typically polyethylene and/or polypropylene, is melted in a heated mould and in which there is typically no pressure applied to cause the plastics material to take up the shape of the mould. Plastics powder is placed in one half of a mould which is then closed and rotated in an oven. Typically the mould itself is rotated biaxially about two perpendicular axes, although in some rotamoulding systems the mould is rotated about one axis inside an oven, and the oven itself and/or the mould is pivoted about a second perpendicular axis (the so-called "rock and roll" rotamoulding system).
As the mould rotates in the oven it heats up, typically to around 200.degree. C., and the plastics material powder starts to melt and coat the inner surface of the mould. Due to the biaxial movement of the mould the powder distributes itself substantially uniformly over the entire inner surface of the mould. When all the powder is melted, the mould is cooled to cause the melted plastics material to solidify in the desired shape. Rotation of the mould is then ceased and it can then be opened to remove the plastic product. The advantages of rotamoulding are increasingly being recognised by manufacturers, including the relatively low cost of moulds, the production of stress-free products (always a problem with injection moulding) and the possibility of novel shapes and designs.
Current rotamoulding machines and processes are relatively labour-intensive. Manual intervention is typically required to open and close the mould and to place the powder in the mould. Moving the mould into and out of the oven and into a cooling station may also be performed manually. Furthermore, due to the biaxial rotation of the mould typically very little control can be exercised over the process within the mould and therefore there is a heavy reliance on experience and trial and error procedures. This latter problem of process control has been overcome to some extent with the development of the rotational moulding process control system developed by The Queens University of Belfast and described in WO 91/0564 ("the ROTOLOG system").
The ROTOLOG system monitors the air temperature inside the mould and transmits this information to a remotely located computer. The real time output from the ROTOLOG system allows the machine operators and quality control personnel to identify the time at which all the powder is melted, the maximum internal air temperature within the mould, the time at which the product has solidified and the heating/cooling rates during the rotamoulding process. Whilst the ROTOLOG system has removed much of the guesswork from the rotamoulding process, it has not reduced the level of manual intervention required. Furthermore, because the ROTOLOG system measures the air temperature within the mould, rather than the actual melt temperature or mould temperature it cannot identify temperature variations on the interior surface of the mould which may have a significant impact on the characteristics of the finished product.